The invention relates to a RF match detector circuit for automatic control of an RF match circuit connected between a power source and a load, such as a plasma in a plasma processing chamber.
In RF plasma processing systems, RF power is typically coupled into the plasma that is present in the chamber through a match circuit which contains variable reactance elements. The purpose of the match circuit is to transform the impedance of the load (i.e., the plasma) to a value that equals or matches the characteristic impedance of the line through which the RF power is delivered to the chamber. At the match point, optimum power is delivered into the plasma and little power is reflected back toward the RF supply. Tuning to the match point is accomplished by appropriately varying the variable reactance elements within the match circuit. Obviously, it is desirable to keep the system tuned to the match point during plasma processing.
Unfortunately, the impedance of the plasma is a complex and highly variable function of many process parameters and conditions. In other words, the impedance of the plasma will typically vary significantly throughout a process run and from one chamber to the next. Thus, to maintain the match circuit at the match point requires constant tuning of the variable reactance circuit components.
Self-tuning circuits are available to perform the required tuning automatically. Unfortunately, many such circuits are not as stable, they do not react to changes in process conditions (i.e., changes in plasma impedance) as rapidly, and they are not as accurate as users (e.g. semiconductor fabrication companies) might wish them to be.